Electronic devices such as integrated circuits are often packaged in hermetically sealed enclosures. These enclosures protect the device from contaminants, particles, and water vapor that would otherwise enter the package and mechanically damage or electrically disrupt the device. The hermetic packages, however, do not perfectly seal out these elements over the life of the device. Additionally, some water vapor and debris is present in the enclosure cavity when the enclosure is sealed, or evolved by the packaging materials as the materials cure.
Getters, compounds that capture contaminants, moisture vapor, and particles, are included inside the device enclosures to trap these species and preclude degradation of device performance, thereby increasing the operational lifetime of the device. Various getter compounds are available depending on the environment to which the getter will be subjected.
Existing getter compounds are unsatisfactory for use with many modern micromechanical devices. Micromechanical devices are small structures typically fabricated on a semiconductor wafer using techniques such as optical lithography, doping, metal sputtering, oxide deposition, and plasma etching which have been developed for the fabrication of integrated circuits. Typical micromechanical devices include accelerometers, pressure sensors, micro-motors, and micromirrors.
Because of their small size, often less than 100 microns, micromechanical devices are very susceptible to damage from debris inside the micromechanical device package. For example, debris can easily obstruct the motion of, or electrically short-circuit, micromirror elements which are often no larger than 17 microns.
Many micromechanical devices include moving components that place unique demands on surface lubrication and passivation systems. For example, the deflectable element of a micromirror device rotates about a torsion beam hinge axis and is stopped by contact with a landing zone or spring structure. The contact point experiences metal-to-metal contact and some scrubbing action. This metal-to-metal contact can create sticking and friction (stiction) between the moving components. Stiction is caused by the capillary action of water vapor present on the surface, van der Waals attraction, and intermetallic bonding of the metals. Stiction becomes worse as the contacting surfaces wear against each other since the contact area is increased.
Passivation coatings on micromirror devices reduce stiction and wear between the contacting surfaces. One passivation material that is especially useful for micromirror devices is a perfluoroalkanoic acid such as perfluorodecanoic acid (PFDA). PFDA, as taught by U.S. Pat. No. 5,331,454, issued Jul. 19, 1994 and entitled Low Voltage Reset for DMD, provides an oriented monolayer on the surfaces of the DMD. The oriented monolayer provides a chemically inert surface that reduces the stiction between adjacent metal parts.
Unfortunately, the PFDA forms a relatively weak bond between with the aluminum surfaces on which it is deposited. Because of the weak bond between the PFDA and the aluminum surfaces of the micromirror device, the scrubbing action between contacting parts wears the oriented monolayer and exposes the underlying aluminum. Without replenishment, exposed aluminum regions grow and eventually create unacceptably large stiction forces—ruining the device. Additional PFDA, however, cannot simply be included in the package since the excess PFDA will condense in crystalline form on the working surfaces of the device, thereby forming particles that obstruct the free operation of the device and causing catastrophic device failure. What is needed is a method and system of replenishing the passivation layer on the micromechanical device while trapping water vapor present in the package that does not introduce particulate PFDA into the package.